Sometimes the most fascinating tourist attractions aren't the most obvious. But if you "dig" a little deeper, you can find rewarding discoveries just under the surface - the earth's surface that is. That's where you'll find Underground California.

With the state's rich mining history and plenty of natural caverns to explore, there does lurk below California a number of attractions that will take you out of the California sunshine just long enough to see there is much more to the Golden State than meets the eye.

Empire Mine Historical Park

Just on the outskirts of the charming historical village of Grass Valley - between Sacramento and Lake Tahoe - we came across the former Empire Mine, now made into a totally captivating state historical park. Set amongst the rolling hills and pine forests of California's Gold Country, the park includes the buildings and mine shafts once used for the state's oldest, largest and richest gold mine.

From 1850 until 1956, when it closed, the Empire Mine produced 5.8 million ounces of gold - surprisingly only about 20 percent of the gold available, with the other 80 percent still there. Also still there are more than 367 miles of abandoned and flooded shafts and tunnels, as well as most of the buildings used during the operation of the mine.

While we couldn't actually go into the mine shafts, we could peer into the entrance of the main tunnel that took miners down a steep incline leading to the web of tunnels underneath. This cold, damp area gives visitors just a hint of the dark, claustrophobic conditions awaiting the miners each day as they traveled beneath ground just to earn a living.

Altogether the park has 847 acres to explore, including buildings, grounds, hiking trails and historical equipment displays that will be sure to enthrall the fix-it person in your household. Elaborate story boards and enlarged photographs are well organized in the park's entrance area where visitors can learn the history of gold mining and understand the importance of the Empire Mine.

The best part is that you'll find the mine in Grass Valley, a fun, historic town liked with Gold Rush-era buildings. We settled in at the Best Western Gold Country Inn - which by the way has a great pool and free breakfast -- and spent the day exploring the area. In many ways the town, although much larger, reminded us of the little mountain town of Julian, California, because Grass Valley seems to be every bit as popular with weekend motorcycle clubs and couples and families just looking for a fun family outing.

Crystal Cave

Next time you visit Sequoia National Park, stop in at the Foothills Visitor Center, near the entrance. When we stopped by, we noticed there was a line at the counter so we figured something was worth waiting for. The visitors in line were signing up for the Crystal Cave, a guided tour that cost $11 for each adult, less for kids, and that was still another several miles' drive into the park. OK, we told the ticket-seller, we're game.

Next came a long, windy drive -- first up into the park, and then down several miles into a canyon to a parking area. At that point we still had a half-mile to walk down the steep pathway, almost to the river below. And then there it was: the entrance to Crystal Cave, mother of all Sequoia caves, or at least the one the public is most invited to explore.

Now this is not just any cave. The entrance is about the size of a railroad tunnel and you'll need a park escort. The guide will take groups of 70 persons or fewer into the cave and through the maze of many adjacent caves and chambers until, quite honestly, we probably never could have found our own way out. I guess this is why, when a little boy had to go to the bathroom, he was escorted back to the entrance by a second tour guide.

Aside from the spookiness that naturally comes from burrowing so far into the earth and its dark inner chambers, the real pay-off on this tour is the natural beauty. All along the way, from "room" to room, we saw gorgeous stalactites and "curtains," as well as ornate marble and all kinds of crystal formations that made this look like some sort of Hollywood movie set. Fortunately, there are paved, lighted pathways that we followed - our guide never far away - and the rooms had just enough illumination to showcase the formations and their brilliance.

On this hot summer day, the 48-degree temperature of the caves was refreshing. Our 45-minute tour was fascinating every step of the way, and climaxed when our tour guide turned off the lights to show us what total darkness was all about: nothing but black and not a thing visible, even an inch in front of your face. Our guide told us there have been times when the power generator has failed so, at this point, we were just about ready to return to the entrance. While many of the rooms are huge, this tour definitely is not for the claustrophobic.

Once the tour was over, it was a relatively steep hike back up to the parking lot. But what an experience -- well worth the price of admission.

Moaning Cavern

We love to explore the many historical attractions near Sonora, Angels Camp and other small towns along Highway 49. Between Angels Camp and the Columbia State Historical Park we came across a unique diversion for those who want an unforgettable experience. We visited Moaning Cavern, just four miles east of Angels Camp, where visitors can walk 100 feet down a spiral staircase to a large cavern. At one time, it was just a hole in the ground that was first discovered by local Indians who would hear a moaning sound coming from the opening. Some would accidentally step into the hole and plunge to their deaths.

When Moaning Cavern was later discovered to be a giant underground cave, many human remains were found. A new opening was cut to allow for the insertion of the staircase and, today, visitors have the choice of taking the staircase or a more adventurous 165-foot rope rappel. If that's still not enough for you, guided tours are offered into the undeveloped and unlighted portions of the cave using lighted helmets and ropes. We chose Option A - the staircase - but there is still something unsettling about being underground in a natural cavern that would be big enough to place the Statue of Liberty inside.

The steep descent and climb back out of the Moaning Cavern have not deterred the thousands of visitors who are only too anxious to explore the mysteries underneath Calaveras County. In fact, there are several other opportunities in the area to go underground, including the California Cavern near Mountain Ranch and Mercer Caverns near Murphys.

While at Moaning Cavern, you might want to check out the new 1,500-foot zip lines. These twin zip lines allow visitors to "race" each other at speeds up to 40 miles per hour over the treetops. It's similar to canopy tours offered in Costa Rica and other exotic locales.

Visitors cross a 60-foot long sky bridge to get to the launch tower, and then they are strapped into a full-body climbing harness and rigged to the cable. You just step off the platform and you're speeding to the landing tour a quarter mile away. The operators say this thrill ride is available to anyone in good health and between 70 and 260 pounds. All you need -- in addition to the price of admission -- is courage.